However I am, obviously, looking for the way of doing it in PE15.
I have a video that is 876x487 and I want to copy out the 400x300 video from the position (x,y): 150,100. The output is to have 400x300 resolution, audio tracks are not be changed and video is not be transformed except probably on the fringes of the selected area (which will become border of the output file).
If I'm understanding you correctly, what you are asking for is not possible, based on how video editing works.
It can be done with photos. In other words, when you crop a photo and save it, you change the size and shape of the photo.
However, when you edit video you are not editing the original video. Your video is source media in your video project. In other words, when you add video to a video project and change it in anyway way, it MUST be rendered and output from the program as a new video, based on the specs and settings of your video project.